It is well established that much of the complex regulation exerted during G1 and S phase traverse in eukaryotic cells is under the control of a family of latent protein kinases, cdks, which are activated by synthesis and binding of their positively acting cyclin subunits. By contrast, substantially less is known about the potential function of cyclin/cdks during terminal differentiation in mammalian cells. Like proliferation, differentiation occurs through a series of tightly controlled temporally regulated sequence of events. However, it is unclear if the same cyclin/cdks that have become an established paradigm in cell growth are the same that are responsible for regulating differentiation We have identified cyclin G2 as a strongly induced gene in murine keratinocytes forced to undergo differentiation in a suspension culture model and in suprabasal keratinocytes harvested from skin. This application seeks to identify the role of this cyclin in enforcing a growth arrest in murine keratinocytes during terminal differentiation. In the first specific aim, the proposed studies will seek to establish whether cyclin G2 functions in collaboration with the cyclin/cdks that exert replicative control in growing cells. These studies will utilize ectopic expression of G1 and S phase cyclins as well as ectopic cyclin G2 expression to ascertain if there is a functional relationship or if cyclin G2 functions through a parallel independent pathway. The studies described in the second specific aim will focus on identifying the factor(s) responsible for the transcriptional induction of cyclin G2 that we have discovered in keratinocytes. Additional investigations will determine the role of the p53 homolog p63 in cyclin G2 induction. The final specific aim will be directed at identifying cyclin G2 interacting proteins in order to identify a catalytic partner(s) for cyclin G2. These studies will be accomplished by using biochemical methods to isolate proteins with affinity for recombinant cyclin G2 and also by screening mammalian cDNA libraries using a two hybrid system.